Present practice in railway signaling makes use of track circuits for train detection and for cab signal. Train detection is most often used for wayside signaling to detect the presence or absence of a train or broken rail and display such a signal to an entering train. Cab signaling circuits provide information concerning track operating conditions to the operator on-board the vehicle. In some installations one signal may provide both of these functions. It is also common to use audio-frequency track circuits for train detection and a different audio-frequency for the cab signal. The cab signal audio-frequency is often coded at a rate indicative of the speed command. The cab signal is fed into the rails and received inductively by an antenna or an induction pick-up mounted usually ahead of the lead axle of the train or elsewhere on the vehicle. It is common practice to use a higher code rate for progressively greater speed commands and to remove the cab signal carrier for a stop command. Because the train should at all times respond to the cab signals that it receives, it is common to inject the cab signal into the rails in advance of the train shifting from one track circuit to another. In such a manner there is no time delay in which a cab signal would be unavailable to a moving train as it passes from one block to another. The loss of a cab signal to a moving train even for a momentary period can result in an unnecessary delay in the orderly flow of traffic on the rails. Normally a track circuit consists of a transmitter at one end and a corresponding receiver at the other end of a block. As long as a proper signal is detected by the receiver, the receiver maintains a relay or equivalent device energized. When the signal is shunted by the presence of a train in the block, the receiver causes the relay to de-energize thereby indicating an occupied track circuit. Various track relays are used to form a selection network which picks the code rate which represents the speed at which the train must not exceed. Such relays are usually vital relays of high precision and corresponding cost. To avoid a second train from following a first vehicle into a block it has been the practice to use vital track relays to cut off the cab signals in the track circuit immediately behind the train. The relay circuitry therefore has provided a means in which cab signals are applied to the block in which the train is located, while removing the cab signal from the block behind the moving vehicle. There would be a significant economic advantage to using a microprocessor controlled logic to perform the speed command selection function and also eliminate the vital relays which are presently used in the cab signal circuitry. Present vital microprocess systems are too slow as a means of quickly applying the cab signal at the entrance to the track circuit. This slow response could cause a momentary loss of cab signal at track circuit boundaries and result in an undesirable train operation. The train operator could be given a stop signal.